1. Field of the Invention
This invention relates to electrochemical cells or batteries and more particularly to rechargeable zinc halogen cells or batteries for example to such cells or batteries in which the electrolyte is primarily zinc chloride but may include traces of iodide and/or bromide, and other impurities such as ammonium-ion compounds, or to such cells in which the electrolyte is bromine chloride or iodide chloride.
2. Description of prior art
A zinc halogen rechargeable cell and a battery built up with such cells has been described in United Kingdom patent specification No. 1,258,502 in which the cathode is of porous carbon forming a reservoir for the halogen acting as the cathode, the porous carbon being supported on a metal or mixture of metals of Group IVA or Group VA of the Periodic Table according to Mendeleef which also acts as a connector in the battery electric circuit and the anode is of zinc on a similar metal support which also acts as a connector, the electrolyte being of the halogen zinc salt solution. In such cells and batteries it has been found that using chlorine as the halogen is a suitable form for a number of reasons including the fact that chlorine is the easiest halogen to handle as well as being the least expensive of the three halogens chlorine, iodine and bromine.
It has been found that titanium rapidly acquires a coating of titanium dioxide which is a non-electrical conductor and hence its use in this condition precludes the use of the titanium in a zinc halogen cell which is to be rechargeable.
It has been found that in cells or batteries of this type commercial zinc chloride on the market has tended to lead in use to a rapid change in the pH value of the electrolyte which thus becomes extremely acidic. It has been shown experimentally that the efficiency of zinc plating of the anode falls off sharply at a pH of less than 1.0; also the coulombic efficiency of the cell or battery is adversely affected and the plating quality if unsatisfactory tending to form dendrites which are highly disadvantageous since they disrupt operation of the cell or battery when the cell or battery is rechargeable frequently after discharge. This undesirable tendency is thought to be due to the impurities in the commercial zinc chloride on the market. It is known that ammonium and iodide ions in the electrolyte both produce runaway pH, and bromine ions do so to a lesser extent.
NH.sub.4.sup.+ and I.sup.- are detrimental above predetermined limits. Thus the accompanying drawing shows the change in pH values in a Zn Cl cell e.g. the change in pH values in a solution of zinc chloride produced from AnalaR reagents, by the controlled addition of NH.sub.4.sup.- or I.sup.- ions. It can be seen from the drawing that a pH of 1.0 is reached with impurity levels of 0.035% NH.sub.4.sup.+ and 0.185% I.sup.-. These are regarded as being the maximum impurity levels for satisfactory operation of the zinc-chloride cell or battery. It is known that other nitrogen compounds e.g. chloramine will have the same effect.
The main object of the present invention is to provide a zinc chlorine cell or battery in which these disadvantages are reduced or minimised.